Preheat the oven to 450F.

Cook the potato halves by placing in a saucepan and filling with enough water to cover the potatoes. Add 1 tsp of salt. Bring to a boil over medium-high heat and cook until the potatoes are tender, about 8 minutes. Drain and set aside.

Place the 1 tbsp of rosemary leaves on the cutting board along with the garlic, 2 tsps of salt and a pinch of red pepper flakes. Mince together. Transfer to a large bowl and add the juice of 1 lemon and the olive oil. Set the used lemon halves aside.

Coat all the chicken pieces with the lemon juice mixture.

Heat a large cast-iron skillet over medium-high heat. Add the chicken, skin-side down, cover and cook until the skin browns, about 5 minutes. Turn the chicken; add the mushrooms and potatoes to the skillet and drizzle with the juice of the remaining lemon.

Add the rosemary sprigs and the squeezed lemon halves to the skillet; transfer to the oven and roast, uncovered, until the chicken is cooked through and the skin is crisp, 20 to 25 minutes.